Materials Science Questions and Answers – Atomic Models

1. In the wave-mechanical model of an atom, degenerate energy levels have:
a) Equal energy
b) Equal number of electrons
c) Same shape
d) Electrons with the same spinView answer

Answer: a
Explanation: Degenerate energy levels have equal energy. In an atom, the orbitals in the same subshell are degenerate in absence of an external magnetic field.

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2. Paired electrons in an atom have equal values of all of the four quantum numbers.
a) True
b) FalseView answer

Answer: b
Explanation: According to Pauli’s exclusion principle, no two electrons in an atom can have all four quantum numbers equal. Paired electrons have opposite spins.

3. Protons attract electrons. Then why do electrons not fall on the nucleus?
a) Neutrons repel the electrons
b) Electrons in ground state cannot radiate energy
c) At very small distances, protons repel electrons
d) Inner electrons repel those in outer orbitalsView answer

Answer: b
Explanation: In the ground state, electrons form standing waves. Standing waves are those in which energy is not transferred. Hence, electrons are bound to revolve around the nucleus with constant potential energy.

Answer: a
Explanation: According to the Aufbau principle, orbitals of lower energy are completely filled before electrons start occupying higher energy orbitals. But in copper, higher energy 3d is filled before lower energy 4s. Such violation of the Aufbau principle appears in about 20 elements.

Answer: b
Explanation: Generally, lower potential energy means more stability. Excited electrons lose energy in the form of photons to shift to stable empty inner shells. Positron is anti-particle of an electron.

7. Around 1911, Rutherford suggested a planetary model of atomic structure. Which of the following was not a proposition of this model?
a) Electrons revolve in circular orbits around the nucleus
b) Nucleus consists of protons and neutrons
c) Mass of atom is concentrated in the nucleus
d) Most of the volume in an atom is voidView answer

Answer: b
Explanation: Neutrons were not discovered by then. Rutherford only suggested that the nucleus has a positive charge, but did not comment on its structure.

8. Which of the Bohr’s postulates was incorrect?
a) Energy of electrons is quantized
b) Momentum of electrons is quantized
c) Electrons release energy when shifting from excited state to ground state
d) Electrons revolve around the nucleus in fixed circular orbitsView answer

Answer: d
Explanation: Heisenberg uncertainty principle implies that exact position and momentum of electrons cannot be known with certainty and hence, definite circular paths are not feasible. Certain regions around the nucleus (called orbitals) have a high probability of the existence of electrons.

9. 4f orbital has higher energy than 5p.
a) True
b) FalseView answer

Answer: a
Explanation: Sum of principal and azimuthal quantum numbers for 4f is (4+3=) 7 and that for 5p is (5+1=) 6. As a general rule, higher the sum, higher is the energy.

10. Which of the following orbitals do not exist?
a) 3p
b) 4s
c) 2s
d) 3fView answer

Answer: d
Explanation: For principal quantum number 3, only possible values of azimuthal quantum number are 0, 1, and 2. Azimuthal quantum number of an f-orbital is 3.